Hydrogen gas is a crucial source of energy, chemically stored in the hydrogen bond, as by far it is the best fuel for proton exchange membrane fuel cells (PEMFC). However, hydrogen gas is difficult to store especially during transportation due to its low energy density per volume and thus hydrogen storage is one of the main challenges in the introduction of fuel cell based power generation.
Ammonia is a carbon free high density hydrogen carrier that does not emit any carbon when decomposed. Only nitrogen, hydrogen and water gases are generated by the decomposition of ammonia.
Ammonia cannot be fed directly to state of the art low temperature fuel cells because ammonia oxidation is extremely slow and inefficient. However, ammonia can be decomposed in a high temperature reactor to generate hydrogen gas.
Unfortunately, the hydrogen gas mixture produced by decomposition of ammonia unavoidably present a concentration of ammonia between few thousand to few parts per million (ppm) of ammonia, due to the nature of the decomposition reaction, i.e. an endothermic equilibrium controlled reaction.
Low temperature fuel cells, e.g. PEMFC, are very susceptible to the ammonia presence in their feed as even the presence of few ppm of ammonia deteriorates the performance of the fuel cell.
Thus, trace amounts of ammonia must be removed from hydrogen gas mixtures in order to use the gas mixtures as a feed for low temperature fuel cells.
A current method for separating traces of ammonia from a fuel cell feed is one that involves the use of a noble metal membrane. The fuel cell feed is flowed through a thin palladium based membrane that separates the ammonia from the hydrogen gas. However, this membrane has the drawback that palladium is very expensive and the membrane produced lack stability. Furthermore, in order to generate a reasonable flow of hydrogen gas, the gas pressure at the inlet side of the membrane has to reach high values, e.g. higher than 8 bar. These values require that the apparatus involved have to withstand high pressure, significantly increasing the costs. In some cases, e.g. when ammonia is released from a metal ammine type solid ammonia storage, these high values are not reachable making the use of a palladium membrane impossible.
Hence, an improved method for separating ammonia from hydrogen gas in hydrogen containing gas mixtures would be advantageous, and in particular a more efficient and/or reliable system for removing ammonia from hydrogen gas in hydrogen containing gas mixtures would be advantageous.